A known drive unit for a movable member includes the movable member adjusting aerodynamic characteristics of a vehicle while the movable member is changing its position, an electric motor operating the movable member, preventing means preventing the movable member from operating beyond a specified operating range by contacting with the movable member, and controlling means supplying an electric power to the electric motor for a reference time to thereby operate the movable member from a first end to a second end of the specified operating range.
A drive unit for a movable fin is provided at a radiator grille for a vehicle as an example of the aforementioned drive unit for the movable member. The movable fin serves as an example of the movable member adjusting aerodynamic characteristics of the vehicle while changing a position. The movable fin adjusts an air penetration to an engine compartment from a front portion of the vehicle that is being driven, thereby applying an air resistance, a lift force, or a down force to the vehicle. A stable driving state of the vehicle is thus ensured. The movable fin is rotatable within a specified operating range defined by a fully closed position (i.e., one of a first end and a second end) where the air penetration to the engine compartment is prohibited and a fully open position (i.e., the other one of the first end and the second end) where the air penetration to the engine compartment is permitted. The rotation of the movable fin beyond the specified operating range is prevented by preventing means that make contact with the movable fin. Without an installation of a sensor that detects the rotation of the movable fin from the first end to the second end of the specified operating range, the movable fin is rotatable from the first end to the second end by the power supply to the electric motor for a reference time (for example, see JP2008-6855A). However, in a case where the reference time is short, the electric motor may stop while the movable fin is rotating within the specified operating range. Therefore, in order to ensure the rotation of the movable fin from the first end to the second end of the specified operating range, the reference time while the electric motor is being powered tends to be longer than a time period actually required for the rotation of the movable fin from the first end to the second end of the specified operating range. Accordingly, even after the completion of the rotation of the movable fin from the first end to the second end, the electric motor is continuously powered until the reference time elapses.
Because the rotation of the movable fin beyond the specified operating range is prevented by the preventing means, a stress may occur in a component of the drive unit. Specifically, the stress may occur in the electric motor, the movable fin, and/or a power transmission mechanism that transmits power from the electric motor to the movable fin by means of the continuous power supply to the electric motor after the completion of the rotation of the movable fin from the first end to the second end of the specified operating range. A smooth operation of the power transmission mechanism may be prevented accordingly. A magnitude of the stress becomes greater in association with an increase of length of a time period while power is being supplied to the electric motor in a state where the rotation of the movable fin is prevented by the preventing means. As a result, a burnout of the electric motor, a breakage of the power transmission mechanism, the movable fin, and the like may occur.
A need thus exists for a drive unit for a movable member which is not susceptible to the drawback mentioned above.